High rate transmit diversity transmission and reception

ABSTRACT

The performance and symbol rate in a wireless mobile system are increased by forming a transmission code matrix using transformed orthogonal codes, in such a way that the code is robust to channel statistics and operates well in both Ricean and (correlated) Rayleigh channels. Furthermore, the invention enables high symbol rate transmission using multiple transmit antennas, and one or multiple receive antennas, and obtains simultaneously high diversity order and high symbol or data rate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. provisional application60/346,482 filed Jan. 4, 2002, which application is hereby incorporatedby reference.

FIELD OF THE INVENTION

This invention relates to methods and systems for achieving joint highdata transmission and diversity in both space and time/frequency in atelecommunication system.

BACKGROUND OF THE INVETION

Signal transmission in wireless communication systems is subject tofading which often reduces the achievable throughput and data rates orachievable quality-of-service. Transmission environments with obstacleslead to multi-path signal propagation, and the power of combinedeffective received signal power can diminish reducing the link capacitysignificantly. In addition, due relative speed between the transmitterand the receiver, or the intermediate objects between the transmitterand the receiver, the fading changes dynamically in time and space.

A typical countermeasure for a fading channel is to employ receiverdiversity with multiple receive antennas. Multiple receive antennas areoften expensive to implement and subsequently alternative solutions havebeen sought for. Transmit diversity is an alternative solution in whichmore than one transmit antenna is used to transmit the signal. Both ofthese techniques create artificial multi-path channels and theprobability that all channels fail simultaneously is significantlyreduced, thus improving the quality of the received signal.

One transmit diversity solution is disclosed in U.S. Pat. No. 6,185,258to Alamouti et al., which is incorporated herein by reference. TheAlamouti matrix C_(Ala) is shown below in equation (1), with each rowcorresponding to a transmit antenna, or a beam, and each columncorresponding to a symbol period.

$\begin{matrix}{{C_{Ala}\left( {z_{1},z_{2}} \right)} = \begin{bmatrix}z_{1} & {- z_{2}^{*}} \\z_{2} & z_{1}^{*}\end{bmatrix}} & (1)\end{matrix}$

The Alamouti scheme is called a 2 by 2 space-time block code, as itemploys two transmit antennas or beams during two symbol periods. As analternative of time-division, transmitting different columns duringdifferent symbol periods, any other substantially orthogonal division ofthe available transmission resources may be used, e.g. differentfrequency subcarriers or Fourier/wavelet waveforms (space-frequencycode) or different (spreading) codes (space-code-code) may be used. Tostress this multitude of uses of a given code matrix, the term “transmitdiversity code” shall be used for codes of the type discussed above,which may be used when a spatial (antenna or beam) dimension isavailable, together with any substantially orthogonal division of othertransmission resources, including time and bandwidth. The transmitdiversity of the Alamouti code is two, as taught in the U.S. Pat. No.6,185,258. The symbol rate is one, since two symbols are transmitted intwo time slots. The code formed according to equation (1) is orthogonal,in the sense that, when multiplied together with its Hermitiantranspose, it yields a scaled identity matrix. The Hermitian transposeof a matrix A, denoted by A^(H), is the complex conjugate transpose ofA. The transpose of a matrix is derived by reversing the row and columnindices of the matrix. The identity matrix, denoted “I”, is a matrixwith each element on its diagonal equal to unity and all other elementseach to zero. Accordingly, for an orthogonal-based matrix A, it holdsthat A^(H)A=AA^(H)=kI, for some real value k The orthogonality of theAlamouti matrix enables separate decoding of the two symbols, in such away that that symbols do not interfere with each other.

The Alamouti transmit diversity is optimized for channels in which thereis little or no intersymbol interference (ISI) on the channel. ISIdistorts the received signal and exacerbates reception, thus reducingsignal quality. The time delayed signals, also known as temporalmulti-path components, can be also advantageous. In CDMA systems onemay, as an example, employ a separate transmit diversity block codedecoder for each multi-path component, and then combine the output usingany suitable diversity combining method, including as an example equalgain combining, or maximal ratio combining. Alternatively, an equalizermay be used to combine the multi-path propagated signals, and possiblyto simultaneously remove inter-symbol-interference. Lindskog and Paulrajhave proposed in “A Transmit Diversity Scheme for Channels withIntersymbol Interference”, Proc. IEEE ICC2000, 2000, vol. 1, pp.307-311, an orthogonal transmit diversity block code that, unlike theAlamouti code, is effective on ISI channels. This paper is incorporatedherein by reference.

Orthogonal transmit diversity codes suffer from rate limitationproblems, as taught in O. Tirkkonen and A. Hottinen, “Complex space-timeblock codes for four Tx antennas” in Proc. Globecom 2000, San Francisco,USA, November/December 2000, incorporated herein by reference. As anexample, the maximal symbol rate for an orthogonal transmit diversitycode with four transmit antennas or beams is ¾. When the rate loss isnot desired the code orthogonality has to be sacrificed. Indeed, O.Tirkkonen, A. Boariu, A. Hoffinen, “Minimal non-orthogonality space-timecode for 3+ transmit antennas,” in Proc. IEEE ISSSTA 2000, September,NJ, USA, teach one such method (e.g. the ABBA code). In this code thesignal is transmitted in using the transmit diversity code matrix

$\begin{matrix}{C_{NOSTBC} = \begin{bmatrix}z_{1} & {- z_{2}^{*}} & z_{3} & {- z_{4}^{*}} \\z_{2} & z_{1}^{*} & z_{4} & z_{3}^{*} \\z_{3} & {- z_{4}^{*}} & z_{1} & {- z_{2}^{*}} \\z_{4} & z_{3}^{*} & z_{2} & z_{1}^{*}\end{bmatrix}} & (2)\end{matrix}$

It is seen that the code comprises as sub-matrices the Alamouti code.The aforementioned paper is incorporated herein by reference. The codedescribed above yields good performance in a fading channel but due tothe structure of the non-orthogonality, there is an inherent performanceloss in correlated channels or in Ricean channels, where knownorthogonal transmit diversity codes perform better. The performance ofnon-orthogonal codes, exemplified by (2), can be improved by employingpossibly matrix valued constellation rotations, as discussed in O.Tirkkonen, “Optimizing space-time block codes by constellationrotations,” Finnish Wireless Communications Workshop, October 2001,which is incorporated here by reference. The idea is that if the symbolsin different orthogonally encoded blocks, exemplified by the pairs z1,z2 and z3,z4 in (2) are taken from different constellations, theperformance of the code is much improved. This can be realized byconstellation rotations.

A simpler, limited diversity space-time code construction has beenproposed for WCDMA systems. The orthogonal code is called STTD-OTD in3GPP document TSGR1#20(01)-0578, incorporated herein by reference. Itcombines two Alamouti codes in such a way that the symbol rate is one(with four transmit antennas), but so that the system only enjoyslimited diversity order. The transmission code matrix is

$C_{{STTD} - {OTD}} = \begin{bmatrix}z_{1} & z_{1} & z_{2} & z_{2} \\{- z_{2}^{*}} & {- z_{2}^{*}} & z_{1}^{*} & z_{1}^{*} \\z_{3} & {- z_{3}} & z_{4} & {- z_{4}} \\{- z_{4}^{*}} & z_{4}^{*} & z_{3}^{*} & {- z_{3}^{*}}\end{bmatrix}$

With four antennas the diversity order is only two when four is themaximum achievable. It is noted that the STTD-OTD code above containstwo Alamouti blocks, and it can be written using the Alamouti marix,given earlier, after changing the column indices 2 and 3. Alternatively,to obtain essentially the same diversity as with STTD-OTD one maycombine antenna hopping and the Alamouti code, in which case thespace-time matrix is

$\begin{matrix}{C_{{STTD} - {AHOP}} = \begin{bmatrix}z_{1} & {- z_{2}^{*}} & \; & \; \\z_{2} & z_{1}^{*} & \; & \; \\\; & \; & z_{3} & {- z_{4}^{*}} \\\; & \; & z_{4} & z_{3}^{*}\end{bmatrix}} & (3)\end{matrix}$

It is seen that the matrix contains four symbols and occupies four timeslots, and hence the symbol rate is one, although all symbols are nottransmitted from all antennas, thus limiting the achievable diversity totwo.

Transmit diversity block codes have been designed also for parallel highrate transmission over fading channels, as taught by O. Tirkkonen and A.Hottinen, “Improved MIMO transmission using non-orthogonal space-timeblock codes,” in Proc. Globecom 2001, November/December 2001, SanAntonio, Tex., USA, incorporated herein by reference. In this method,two transmit antennas and two receive antennas are used advantageouslyto obtain both transmit/receive diversity benefit and increased data orsymbol rate.

High rate space-time transmission concepts have been considered also forfuture WCDMA systems. Indeed, in the Third Generation PartnershipProgram (3GPP) document “Improved Double-STTD schemes using asymmetricmodulation and antenna shuffling” TSG-RAN Working Group 1(TSGR1#20(01)-0459) by Texas Instruments (incorporated herein byreference), proposed parallel transmission of Alamouti codes using fourtransmit antennas and two or four receive antennas. Although this methodimproves the symbol rate by a factor of two it obtains only limiteddiversity order, which limits the performance and realizable data rates.

SUMMARY OF THE INVENTION

It is an aim of embodiments of the present invention to address one ormore of the problems discussed above.

The present invention provides a method to transmit complex symbolsusing a transmission matrix, said method comprising the steps ofconverting a stream of complex symbols to at least two at leastpartially different streams of complex symbols, a modulating said atleast two streams of complex symbols to form at least two code matrices,at least one of which is of dimension greater than one, transformingsaid code matrices using linear transformations, to construct at leasttwo transformed transmit diversity code matrices, constructing atransmission code matrix using at least two transformed transmitdiversity code matrices, and transmitting said transmission code matrix,at least partially in parallel, using substantially orthogonal signalingresources and at least three different transmit antenna paths.

It is also an object of the invention to provide a method and apparatusfor receiving a signal comprising a channel estimation module thatoutputs estimates of the impulse response estimates from each transmitantenna path to each receive antenna, and a detection module that usesthe structure of a transmission matrix, said matrix comprising at leastone linear combination of two orthogonal space-time code matrices orchannel symbols, and channel impulse response estimates to calculate bitor symbol estimates for transmitted signal stream or streams.

It is further an object of the invention to provide an apparatus totransmit complex symbols using a transmission matrix, said apparatuscomprising conversion means for converting a stream of complex symbolsto at least two at least partially different streams of complex symbols,modulating means for modulating said at least two streams of complexsymbols to form at least two code matrices, at least one of which is ofdimension greater than one, transforming means for transforming saidcode matrices using linear transformations, to construct at least twotransformed transmit diversity code matrices, code constructing meansfor constructing a transmission code matrix using at least twotransformed transmit diversity code matrices and transmission means fortransmitting said transmission code matrix, at least partially inparallel, using substantially orthogonal signaling resources and atleast three different transmit antenna paths.

Other objects and characteristics of the present invention are apparentfrom the detailed descriptions explained in conjunction with the relateddrawings. The drawings are designed only to illustrate the inventiveconcept, and in no way limit the application of the invention, for whichreference should be made to the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 depicts an exemplary transmit diversity system in accordance withthe present invention;

FIG. 2 depicts an exemplary receiver system for the proposed high ratetransmission method

FIG. 3 depicts a multi-antenna transmitter-receiver pair in accordancewith the invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference is made to FIG. 1, which illustrates a high rate transmitterdiversity system which includes a transmitting apparatus 101, such as abase station, and a receiver 102, such as a mobile phone. Thetransmitting apparatus 101 includes serial-to-parallel (S/P) module 103,a plurality of transmit diversity modulation modules for constructingorthogonal matrices 104, at least one transform module 105, codeconstruction module 106, signaling/spreading modules 107 and a pluralityof RF (Radio Frequency) modules that convert the signal to analogwaveforms 108, and transmitting antennas 109. The receiver 102 includespossibly more than one receiving antennas.

The S/P module 103 converts the possibly channel coded (ENC) (e.g. Turbocoded or convolutional coded) and modulated (MOD) complex valued symbolstreams into at least two parallel complex valued symbol streams inwhich at least some of the complex symbols are different from eachother. In modules 104, the two symbol streams are separately transmitdiversity coded, using orthogonal (transmit diversity) code matrices C1and C2, each of arbitrary dimension and rate. According to certainembodiments of the present invention, the symbol rate is the same as anaverage symbol rate of the orthogonal code matrices. The symbols in the(transmit diversity) code matrices C1 and C2, or the (transmitdiversity) code matrices themselves are mixed using a lineartransformation U in module 105, to generate transformed transmitdiversity code matrices X1 and X2. The transformed code matrices X2 andX2 are used to construct a transmission code matrix in C the codeconstruction module 106. The transmission code matrix is transmittedusing at least three transmit antennas or paths or beams 109 using anysubstantially orthogonal signaling, e.g. different orthogonal codes(e.g. Hadamard codes) or different time slots or different Fourierwaveforms (OFDM waveforms/subcarriers) or different frequency bands canbe used. If optional parallel transmission is used to increase thesymbol rate the S/P module can output more than two symbol streams 110,to be eventually combined with other parallel streams or code matricesin code construction module 106.

As an example, the transformed code matrices can be formed in thetransform module 105 as follows:X1=C1+C2X2=C1−C2  (4)in which C1 and C2 are two orthogonal transmit diversity codes, e.g. the2 by 2 Alamouti matrices. It should be stressed that normalization, tomaintain target symbol energy by diving by √{square root over (2)}, hasbeen omitted for simplicity.

To further improve performance, the symbols in the respective orthogonaltransmit diversity codes C1, C2 may be taken from different modulationalphabets, effected e.g. by constellation rotations. This may beincorporated in the serial-to-parallel module (103), which may be moregenerally considered as a conversion means for converting a stream ofcomplex symbols to multiple streams of complex symbols. Alternatively,or in addition, the transform module 105 may be generalized so that thetransmission matrices are be formed using a unitary transformationmatrix U, asX=UC  (5),where (assuming only two code matrices are transformed)

$C = \begin{bmatrix}{C1} \\{C2}\end{bmatrix}$and U is for example, of the formU=V{circle around (×)}Iwhere V is a Unitary matrix and I is an identity matrix. Matrix V mayassume the parameterized form

$V = {\begin{bmatrix}\mu & \gamma \\{- \gamma^{*}} & \mu^{*}\end{bmatrix}.}$

The Kronecker product, when combined with the identity matrix, maximizesthe number of zero elements on the transformation matrix, and thisreduces the peak-to-average ratio in the transmitter and provides asimple implementation. We may also further parameterizeμ=√{square root over (α)};γ=√{square root over (1−α)}exp(−jφπ)where α determines the relative powers within the linear combinationmatrix and φ a determines the phase of a complex phasor. The simpletransformation (4) is obtained by setting α=½,φ=0.

Return for simplicity to the special case provided in equation (4). Thecode construction module 106 takes the transformed transmit diversitycode matrices X1 and X2 resulting form the linear combinations andconstructs a transmission code matrix. For this, the code constructionmodule 106 embeds the matrices X1 and X2 into a transmission code matrixwith double the dimensions. The transformed code matrices canadvantageously be transmitted in different time slots (time-orthogonalsignaling) using four transmit antennas using a transmit diversitytransmission code matrix

$\begin{matrix}{C_{{TR} - {AHOP}} = {\begin{bmatrix}{X1} & \; \\\; & {X2}\end{bmatrix}.}} & (6)\end{matrix}$

Alternatively, to maintain better power balance one may transmitcontinuously using e.g. the transmit diversity transmission code matrix

$\begin{matrix}{C_{{Tr} - {OTD}} = \begin{bmatrix}{X1} & {X2} \\{X1} & {- {X2}}\end{bmatrix}} & (7)\end{matrix}$

These transmission code matrices can be subjected e.g. to column and/orrow permutations without affecting the code properties. The transmissioncode matrices may also be subjected to multiplying from the left and/orwith a constant matrix.

To understand the benefits of the use of said linear (unitary)combinations, recall that the received signal (for simplicity in a flatfading channel) can be described as, y=Ch+n, where h is the vector ofchannel coefficients to a given receive antenna. An equivalent modelsignal model follows, when symbols within X1 and X2 and the channelmatrix/vector are rearranged, y′=Hb+n′, where H depends on the codematrix and the channel, b is a symbol or a bit vector, where differentdimensions correspond to different transmitted symbol or bit streams.When symbols are QPSK modulated with Gray labeling, b may be consideredto comprise I and Q components of each symbol, thus increasing thedimensionality of the vector by a factor of two.

The received equivalent correlation matrix, which can be used whendetecting the symbols or bits, follows, when a space-time matched filter(i.e., conjugate transpose of H) is applied to the received signal y′,to form a equivalent signal model after space-time matched filterz=Rb+n″, where R is called an equivalent channel correlation matrix.This model can be used be the corresponding receiver e.g. by estimatingthe symbols or bits by minimizing (wrt. b)∥z−Rb∥²,possibly under colored noise or, as an alternative, one may solvedirectly∥y−Hb∥²,where, as above, H is the equivalent channel matrix that depends on thetransmission matrix. Detectors solving these equations are well-known.However, in order to arrive at these models we need to make sure themodel matches the properties of channel vector/matrix and transmissioncode matrix. As an example, When estimating bits or symbols using theequivalent channel correlation matrix, we need to know said matrix. Forexample, for codes (6) and (7), with α=½,φ=0, and with four transmitantennas, this results in code correlation matrix with structure

$\begin{matrix}{{R = {{aI}_{NT} + {\begin{bmatrix}0 & b \\b & 0\end{bmatrix} \otimes I_{2}}}}{with}} & (8) \\\begin{matrix}{a = {\sum\limits_{j = {1:{NR}}}^{\;}{\sum\limits_{i = {1:{NT}}}^{\;}{h_{ij}}^{2}}}} \\{b = {{\sum\limits_{j = {1:{NR}}}^{\;}{\sum\limits_{i = {1:{{NT}/2}}}^{\;}{h_{ij}}^{2}}} - {\sum\limits_{j = {1:{NR}}}^{\;}{\sum\limits_{i = {{{{NT}/2} + 1}:{NT}}}^{\;}{h_{ij}}^{2}}}}}\end{matrix} & (9)\end{matrix}$

Thus, the code above is non-orthogonal since the off-diagonalcorrelation values do not vanish. Correspondingly when α=1, φ=0 (in thisspecial case the transmission code matrix reduces to STTD-OTD, knownfrom prior art)

$R = \begin{bmatrix}{a_{1}I_{2}} & 0 \\0 & {a_{2}I_{2}}\end{bmatrix}$ where $\begin{matrix}{a_{1} = {\sum\limits_{j = {1:{NR}}}^{\;}{\sum\limits_{i = {1:{{NT}/2}}}^{\;}{h_{ij}}^{2}}}} \\{a_{2} = {\sum\limits_{j = {1:{NR}}}^{\;}{\sum\limits_{i = {1:{{{{NT}/2} + 1}:{NT}}}}^{\;}{h_{ij}}^{2}}}}\end{matrix}$

This reflects the fact that different symbols obtain only partialtransmit diversity, whereas in (8)-(9) the diagonal elements areidentical, and thus all symbols obtain same power regardless of thechannel realization. The equivalent correlation matrix can be used whendetecting the symbols or bits. It is essential to notice that the codecorrelation matrix of the proposed code differs from that of prior art(e.g. ABBA) so that the former depends explicitly on channel powerdifferences, while the latter depends on complex phases of the channel.This property of the inventive code can be used to advantage, whenincreasing the data rates of the transmission quality of thetransmission system. In particular, the invented code is orthogonal whenthe channel is fully correlated, and in general the correlationcoefficient diminishes as the channel correlation increases. Therefore,the code is suitable also for correlated channels, that can be describede.g. as correlated Rayleigh or Ricean channels. Conversely, the priorart non-orthogonal code (ABBA) remains non-orthogonal in these channels.The correlation properties of the physical channel depend on theenvironment, but it is generally known that antenna correlationincreases when the transmitting or the receiving antennas are close toeach other. With small base stations and especially with small terminals(mobile stations) this is likely the case in the future wirelesssystems.

To recapitulate, a general linear transformation matrix U, holds theaforementioned transformation (4), and the prior art code (known asSTTD-OTD with 4 transmit antennas) as a special case: Transformation (4)is obtained by setting α=½, φ=0, and STTD-OTD code by setting α=1. When½<α<1 the code provides smaller transmit diversity benefit, as thediagonal values of the code correlation matrix differ from each othermore than in Equation (8). The advantage is that the code correlationvalues are smaller in magnitude. This simplifies detection in thereceiving unit. In the extreme case, the code reduces to an orthogonalSTTD-OTD like code, in the sense that the code correlation values(off-diagonal values of the code correlation matrix) are zero. It shouldbe noted that the correlation matrix of the proposed code also leads toadvantageous properties when the code is used on properly equalized ISIchannels; the multipath components attenuate the non-orthogonality ofthe code. For this, the individual symbols may be interpreted as vectorsof multiple symbols.

It is noted that when the constituent matrices C1 and C2 are bothAlamouti matrices, the obtained code has symbol rate at least one (atleast four different symbols can be transmitted in four time slots). Theinventive transmission concept can also be used when increasing thesymbol rate of the transmitting device. In this case several transformedtransmit diversity code matrices are transmitted in parallel, preferablycontinuously to minimize power fluctuations in the transmitter (i.e. tominimize peak-to-average ratio in the RF power amplifiers). To thiseffect, a particularly advantageous embodiment is to fill up theanti-diagonal part of the TR-AHOP matrix above as follows

$\begin{matrix}{{C_{{2{TR}} - {AHOP}} = \begin{bmatrix}{X\; 1} & {X\; 3} \\{X\; 4} & {X\; 2}\end{bmatrix}},} & (10)\end{matrix}$in which the matrices X3 and X4 are formed analogously using lineartransformations. Advantageously, the linear transformations used toconstruct X3 and X4 are different from the ones used to construct X1 andX2. For example, the modulation alphabets of symbols used to constructX3 and X4 may be different than the modulation alphabets of symbols usedto construct X1 and X2. In effect, four parallel complex symbol streamsare formed, a transformation is applied to two streams separately, andthe two transformed transmit diversity code matrices are transmittedsimultaneously and continuously from the transmitting devices, such thatpair X1 and X2, and pair X3 and X4 both obtain full diversity benefit,and typically interference from each other.

The invention is in not way limited to the use of the Alamouti transmitdiversity code as submatrices of the transformed code. In general, anyorthogonal transmit diversity code defined for any number of transmitantennas can be used. All possible orthogonal transmit diversity codematrices were constructed in the patent application, WO/63826 A1, whichis incorporated herein by reference. As an example codes C1 and C2 canbe rate ¾ transmit diversity codes defined each for four transmit pathsor antennas, in which case the resulting code is defined for eighttransmit paths or antennas, with overall symbol rate ¾. Hence the rateis increased when compared to orthogonal transmit diversity codes, inwhich rate ½ cannot be exceeded using eight antennas, as is well knownfrom prior art. Alternatively, code C2 may be defined (as taught inprior art) to have e.g. rate ½, while C1 has rate ¾ in which case theoverall rate is ⅝.

Thus, the space time-code matrices C1 and C2 need not be the same, toenable a large number of different symbol rates. In addition, they neednot even have the same dimension. Hence, (recalling the fact that thecode dimension depends on the number of antennas), by using differentcode dimensions enables one to divide the transmitting elements(antennas) asymmetrically for the transformed code X1 and X2. As anexample, if C1 is the Alamouti code of dimension two and C2 is rate ¾code of dimension four (Appendix 1, eq. (4)), we have effectively symbolrate ⅞ and 6 antenna transmission, and C2 is defined as

$\begin{matrix}{{C\; 2} = \begin{bmatrix}{C\; 1^{\prime}} & {C\; 2^{\prime}} \\{C\; 1^{\prime}} & {{- C}\; 2^{\prime}}\end{bmatrix}} & (11)\end{matrix}$where C1′ and C2′ are two Alamouti codes (essentially STTD-OTD transmitdiversity transmission matrix) formed with two different symbols. If thedimension is not the same it is understood that either the matrix withsmaller dimension is canonically filled up with zeros when transformingthe codes, or that the larger matrix is punctured (e.g. columnsdeleted), to allow the use of arbitrary number of transmit antennas,e.g. 6 in the above exemplary case.

FIG. 2 depicts a corresponding receiver, consisting of one or aplurality of receive antenna, and RF front-end modules 201, anddespreading or channel-division units 202, that convert the signal tobase band in which the space-time matched-filters and channel estimatesprovided by 203 together with suitable detection device 204 fordetecting the symbols or bits in embedded in the transmission codematrix. The channel estimation unit 203 determines the complex channelcoefficient for each transmit antenna-receive antenna pair. In apreferred embodiment the receiving unit uses the channel estimates (andperhaps also signal-to-noise ratio estimates) to construct the effectivecorrelation matrix depending on the used transformation matrix. Anexplicit example for Transformation (4) was shown in equations (8) and(9) above. The detection device 204 can be any decoder or a jointdecoder and channel estimation unit, e.g. formingMinimum-Mean-Square-Error (MMSE) estimates of the transmitted symbols,or Maximum Likelihood estimates e.g. by a Viterbi algorithm, orsoft-outputs (a posteriori probabilities) by an optimal or suboptimalMAP algorithm. Joint detection is enabling by allowing feedback andfeedforward connections via 206 between the outputs delivered bydetection, channel estimation, channel decoding units. Eventually, thechannel decoder (e.g. Turbo decoder) forwards the decisions to otherreceiver units or to the targeted sink of the particular the source.These receiver concepts are generally well known, but when applied inthe context of the present invention the receiver can utilize the codestructure used in the transmitter, and possibly also the codecorrelation matrix similar to that of equation (8), for example.

While there have been shown and described and pointed out fundamentalnovel features of the invention as applied to preferred embodimentsthereof, it will be understood that various omissions and substitutionsand changes in the form and details of the methods described and devicesillustrated, and in their operation, may be made by those skilled in theart without departing from the spirit of the invention. For example, itis expressly intended that the method can be used with any substantiallyorthogonal signaling when transmitting the transformed transmitdiversity code matrices over the channel. This includes the use of OFDMwaveforms, wavelets, time-orthogonal waveforms, FDMA, and arbitrarysubstantially orthogonal spreading codes. Furthermore, the complexsymbols modulating the entries of the transmit diversity code matricescan be from arbitrary complex modulation alphabets (QPSK, 8-PSK, 16-QAM,4-PSK), or (possibly matrix valued) constellation rotated versions ofthese, and that different alphabets can be used for different streams ofcomplex symbols. Also, the bits that are encoded in the complex symbolsmay be channel coded and/or interleaved. The channel code may be a blockcode, a trellis code, a convolutional code, a turbo code, a low-densityparity check code, or any combination of these or any other code knownin the art. The interleaver may be a bit, symbol or coordinateinterleaver. The transmit paths to which the columns of the transmitdiversity code matrix are transmitted can be formed by beams using atleast two transmitting elements, or fixed beams using e.g. a Butlermatrix, or by using beams controlled by the received using some feedbackloop, or by any other method known in prior art to form a transmit pathor transmit pats from the transmitter to the receiver. In addition, itis clear that although the method of the invention requires only onechannelization waveform or code, if more than one are available the datarate can be directly increased using multicode transmission.Furthermore, it is considered apparent that the channel estimatesrequired to detect the streams of complex symbols can be obtained e.g.by using common channel pilots, or dedicated channel pilots, or by usingdecision feedback, or any combination thereof. Also, for ISI channels itis possible to transmit symbol-vectors instead of symbols, withappropriate interpretations of the complex-conjugations, without partingfrom the scope of this invention. It is the intention, therefore, to belimited only as indicated by the scope of the claims appended hereto.

1. A method, comprising: constructing a transmission code matrix, andtransmitting said transmission code matrix, at least partially inparallel, using substantially orthogonal signaling resources and atleast three different transmit antenna paths, wherein said transmissioncode matrix can be expressed as being constructed using at least twotransformed transmit diversity code matrices, wherein the two matrixdimensions of said transmission code matrix are greater than thecorresponding matrix dimensions of said transformed transmit diversitycode matrices, wherein said transformed transmit diversity code matricescan be expressed as being constructed by transforming at least twotransmit diversity code matrices using linear transformations, andwherein said transmit diversity code matrices, at least one of which isof dimension greater than one, can be expressed as being formed bymodulating at least two at least partially different streams of complexsymbols that are obtainable from a single stream of complex symbols byconversion.
 2. The method of claim 1, wherein constructing saidtransmission code matrix comprises: converting a stream of complexsymbols to at least two at least partially different streams of complexsymbols; modulating said at least two streams of complex symbols to format least two transmit diversity code matrices, at least one of which isof dimension greater than one; transforming said transmit diversity codematrices using linear transformations, to construct said at least twotransformed transmit diversity code matrices; and constructing saidtransmission code matrix comprises using said at least two transformedtransmit diversity code matrices.
 3. The method of claim 1, wherein atleast one of the linear transformations is different from an identitytransformation.
 4. The method of claim 1, wherein the at least twotransmit diversity code matrices are orthogonal transmit diversity codematrices.
 5. The method of claim 4, wherein the symbol rate of thetransmission code matrix is the same as an average symbol rate of theorthogonal transmit diversity code matrices to which the lineartransformations are applied.
 6. The method of claim 1, wherein thetransmission code matrix can be expressed as being constructed from thetransformed transmit diversity code matrices using the method ofembedding a lower-dimensional matrix into a higher-dimensional one. 7.The method of claim 1, wherein the transmission code matrix can beexpressed as being constructed from the transformed transmit diversitycode matrices using at least one of the methods of repetition, negation,conjugation, permutation, multiplying with a matrix.
 8. The method ofclaim 1, wherein the first transformed transmit diversity code matrixcan be expressed as being constructed by summing two transmit diversitycode matrices, and the at least the second transformed transmitdiversity code matrix can be expressed as being constructed bysubtracting the said two transmit diversity code matrices.
 9. The methodof claim 1, wherein the transmission code matrix extends over Tsubstantially orthogonal signaling resources, and wherein more than Tcomplex symbols are used to construct the transmission code matrix,wherein T is equal to at least one.
 10. The method of claim 9, whereinat least two transformed transmit diversity code matrices aretransmitted in parallel, and wherein the at least two transformedtransmit diversity code matrices contain at least partially differentsymbols.
 11. The method of claim 9, wherein a part of the symbols aretransmitted on a block-diagonal sub-matrix within the transmission codematrix, and at least partly different symbols are transmitted on ananti-block-diagonal sub-matrix within the transmission code matrix. 12.The method claim 9, wherein there are four substreams and wherein eachsubstream is modulated to form an orthogonal 2×2 transmit diversity codematrix incorporating two complex symbols, and wherein the transmissioncode matrix extends over at least four substantially orthogonalsignaling resources.
 13. The method of claim 1, wherein said conversioncomprises a serial-to-parallel conversion.
 14. The method of claim 1,wherein said conversion comprises a rotation unit.
 15. The method ofclaim 14, wherein the rotation unit is a symbol rotation matrix thatdiffers from an identity matrix, and contains at least twozero-elements.
 16. The method of claim 15, wherein the symbol rotationmatrix is a diagonal matrix, where at least one diagonal element is acomplex number.
 17. The method of claim 14, wherein the rotation unit isa symbol rotation matrix that is formed as Kronecker product of twounitary matrices, where at least one is different from an identitymatrix.
 18. The method of claim 1, wherein at least one transmitdiversity code matrix has a different symbol rate than another transmitdiversity code matrix.
 19. The method of claim 1, wherein at least onetransmit diversity code matrix has different dimensions than anothertransmit diversity code matrix.
 20. The method of claim 1, wherein atleast one transmit diversity code matrix is transmitted with differentpower than another transmit diversity code matrix.
 21. The method ofclaim 1, wherein the substantially orthogonal signaling resourcesinclude at least one of the following: non-overlapping time slots,different spreading codes, different OFDM subcarriers, different waveletwaveforms and different FDMA channels.
 22. The method of claim 1,wherein said transmission code matrix can be expressed as beingconstructed using two transformed transmit diversity code matricesresiding on the block-diagonal of said transmission code matrix, whereinone of said transformed transmit diversity code matrices can beexpressed as being constructed as a sum of two transmit diversity codematrices, and wherein the other transformed transmit diversity codematrix can be expressed as being constructed as a difference of said twotransmit diversity code matrices, and wherein said two transmitdiversity code matrices can be expressed as being formed by modulatingtwo different streams of complex symbols that are obtainable from asingle stream of complex symbols.
 23. An apparatus, comprising: aprocessor configured to construct a transmission code matrix out ofcomplex symbols; and a transmission unit configured to transmit saidtransmission code matrix, at least partially in parallel, usingsubstantially orthogonal signaling resources and at least threedifferent transmit antenna paths, wherein said transmission code matrixcan be expressed as being constructed using at least two transformedtransmit diversity code matrices, wherein the two matrix dimensions ofsaid transmission code matrix are greater than the corresponding matrixdimensions of said transformed transmit diversity code matrices, whereinsaid transformed transmit diversity code matrices can be expressed asbeing constructed by transforming at least two transmit diversity codematrices using linear transformations, and wherein said transmitdiversity code matrices, at least one of which is of dimension greaterthan one, can be expressed as being formed by modulating at least two atleast partially different streams of complex symbols that are obtainablefrom a single stream of complex symbols by conversion.
 24. The apparatusof claim 23, wherein said components configured to construct atransmission code matrix comprise: a converter configured to convert astream of complex symbols to at least two at least partially differentstreams of complex symbols; a modulator configured to modulate said atleast two streams of complex symbols to form at least two transmitdiversity code matrices, at least one of which is of dimension greaterthan one; a transformer configured to transform said transmit diversitycode matrices using linear transformations, to construct said at leasttwo transformed transmit diversity code matrices; and a code constructorconfigured to construct a transmission code matrix using said at leasttwo transformed transmit diversity code matrices.
 25. The apparatus ofclaim 24, wherein at least one of the linear transformations isdifferent from an identity transformation.
 26. The apparatus of claim24, wherein the at least two transmit diversity code matrices areorthogonal code matrices.
 27. The apparatus of claim 23, wherein saidtransmission code matrix can be expressed as being constructed using twotransformed transmit diversity code matrices residing on theblock-diagonal of said transmission code matrix, wherein one of saidtransformed transmit diversity code matrices can be expressed as beingconstructed as a sum of two transmit diversity code matrices, andwherein the other transformed transmit diversity code matrix can beexpressed as being constructed as a difference of said two transmitdiversity code matrices, and wherein said two transmit diversity codematrices can be expressed as being formed by modulating two differentstreams of complex symbols that are obtainable from a single stream ofcomplex symbols.
 28. A system, comprising: processor configured toconstruct a transmission code matrix out of complex symbols, and atransmitter configured to transmit said transmission code matrix, atleast partially in parallel, using substantially orthogonal signallingresources and at least three different transmit antenna paths, whereinsaid transmission code matrix can be expressed as being constructedusing at least two transformed transmit diversity code matrices, whereinthe two matrix dimensions of said transmission code matrix are greaterthan the corresponding matrix dimensions of said transformed transmitdiversity code matrices, wherein said transformed transmit diversitycode matrices can be expressed as being constructed by transforming atleast two transmit diversity code matrices using linear transformations,and wherein said transmit diversity code matrices, at least one of whichis of dimension greater than one, can be expressed as being formed bymodulating at least two at least partially different streams of complexsymbols that are obtainable from a single stream of complex symbols byconversion.
 29. The system of claim 28, wherein said transmission codematrix can be expressed as being constructed using two transformedtransmit diversity code matrices residing on the block-diagonal of saidtransmission code matrix, wherein one of said transformed transmitdiversity code matrices can be expressed as being constructed as a sumof two transmit diversity code matrices, and wherein the othertransformed transmit diversity code matrix can be expressed as beingconstructed as a difference of said two transmit diversity codematrices, and wherein said two transmit diversity code matrices can beexpressed as being formed by modulating two different streams of complexsymbols that are obtainable from a single stream of complex symbols.